1. Field of the Invention
The invention relates to a device for operation of a high pressure discharge lamp and a projector device.
2. Description of the Prior Art
The types of projector devices generally include one that uses a liquid crystal cell and the DLP (digital light processing) type.
In the type using a liquid crystal cell, there are the single sheet type and the three-sheet type. In each type, the following takes place:                the radiant light from the light source is separated into three colors (RGB);        the light which corresponds to the video information is transmitted and regulated by a liquid crystal cell;        afterwards, the three colors which have been transmitted by the cell are combined and projected onto the screen.        
On the other hand, in the type using DLP the following takes place:                the radiant light is emitted from the light source via a rotary filter in which RGB regions are formed by division onto a space modulation element (also called a light modulation device—specifically defined as a DMD (digital micro mirror device) element or the like) or the like, by time division; and        certain light is reflected by the DMD element and is emitted onto a screen.        
Here, the expression “DMD element” is defined as follows:
Each pixel is covered with a few hundred million small mirrors at a time. Light projection is controlled by controlling the direction of the individual small mirrors.
The DLP type, as compared to the liquid crystal type, has the advantage that the overall device can be small and simple because the optical system is simple and because it is not necessary to use as many as three liquid crystal cells. High pressure discharge lamps, especially super-high pressure mercury lamps, are often used for the light sources of these projector devices.
FIG. 5 shows a general circuit diagram of an alternating current discharge lamp. The circuit has a direct current source VDC, a voltage reduction chopper circuit 1 which has a switching device Qx, a full bridge circuit 2 which has switching devices Q1 to Q4, and a starter circuit 3 for starting operation.
The circuit operates as follows:
By supplying voltage and current from the direct current source to the full bridge circuit 2, the switching devices Q1, Q4 and the switching devices Q2, Q3 of the full bridge circuit 2 are turned on in alternation, and an alternating current voltage with rectangular waves is supplied to the discharge lamp 10.
The high pressure discharge lamp is operated in this way.
When the discharge lamp 10 is started, a high voltage pulse is applied by the above described starter circuit 3 to the discharge lamp 10, and thus the lamp is started.
In the full bridge circuit 2, there is a time which is the so-called dead time and in which all switching devices Q1 to Q4 are turned off. The reason for this is to prevent a cross current and damage to the switching devices Q1 to Q4.
Even when the dead time has expired and some switching device is being turned on, the desired amount of lamp current does not flow immediately, but, as shown using a gentle rising waveform, it gradually increases and reaches the desired amount. The reason for this is that, between the output of the full bridge circuit 2 and the discharge lamp, there are a coil L1, a capacitor C1 and a transformer Tr1.
This gentle rising of the lamp current becomes a major factor for reducing the light intensity of the discharge lamp because the lamp current is not 0, but an insufficient feed amount.
This origin of the dead time and the gentle rise of the lamp current after expiration of the dead time do not cause any special disadvantage for a general application. The reason for this is that the light dip due to the dead time and to the gentle rise of the following current is roughly 100 μsec and that no influence is exercised in practice as the fluctuation of the light intensity.
However, in a projector device of the gradual chromatic type using a space modulation element and a rotary color filter and the like, there is a fatal disadvantage associated with the indicated effect. The reason for this is that, in the case of using a space modulation element and a rotary filter, even when the light intensity fluctuates with an amount of several 10 μsec, a major influence can be exerted on the precision of the projected image (compare JP-A SHO 62-26796).